The present invention relates to printed circuit boards of the type having a solder mask over non-reflowable metal, such as bare copper traces etched on a suitable substrate, and, more particularly to a method of manufacture of such circuit boards and to the unique printed circuit boards resulting therefrom.
As is well understood in the art, the manufacture of double-sided printed circuit boards requires the provision of conductive through-holes for interconnecting components on opposite sides of the board. The non-conductive surfaces exposed when through-holes are drilled in a non-conductive substrate having metal cladding on both sides must, therefore, be provided with a conductive coating, and this generally is accomplished by a first electroless deposition of copper onto the suitably conditioned through-hole surfaces, followed by electroplating of copper to build up additional thickness.
In application of the actual circuit patterns to the metal-clad board surfaces, it is necessary to employ plating resists so as to prevent all but particular areas of the board (through-holes and/or traces and/or pads and/or other areas) from receiving applied metal platings such as the copper electroplate used in through-hole plating or the commonly-employed tin-lead coating applied as an etch-resist preliminary to the step of etching away undesired metal down to the non-conductive substrate surface so as to form the appropriate conductive circuit pattern.
Apart from its use as an etch-resist, tin-lead is a preferred overplating for otherwise exposed copper areas on the circuit board so as to prevent oxidative degradation of the copper surfaces.
In the ultimate fabrication of a printed circuit board in which various components and connections are soldered, it is generally accepted that improved solderability of circuit pads and through-holes can be provided to the ultimate fabricator by having the manufacturer precoat these areas with a solderable metal, generally a tin-lead composite closely similar in composition to the solder actually used in the eventual soldering of components and connections. For applications where hand-soldering by the fabricator is to be performed, little difficulty is encountered in applying solder to desired areas without disturbing or inadvertently soldering adjacent conductive traces. However, when soldering is to be conducted in mass techniques, such as with wave soldering or dip soldering methods, inadvertent soldering and improper connections can occur. As a consequence, manufacturers apply a solder resist or solder mask over those areas of the board to be protected from solder, including the tin-lead coated copper traces.
It has been recognized that the technique of solder resist over tin-lead coated copper can, however, lead to its own peculiar problems. For example, since the tin-lead is a reflowable metal, ultmate wave or dip soldering can cause the tin-lead to wick up under the mask or simply to melt and no longer provide support for the mask. Due to these disadvantages, it has been proposed to apply the solder mask directly over bare copper at those areas where protection from solder is desired. This "solder mask over bare copper" (SMOBC) technique avoids the problems inherent in the application of the mask over tin-lead coated copper, and can yield printed circuit boards with finer line definition and higher current density capabilities. Unfortunately, the known solder mask over bare copper techniques involve added manufacturing operations, and hence added cost, and present waste disposal and pollution control problems.
In order to explain these disadvantages in more detail, a typical SMOBC process is schematically set forth in the cross-sections represented by FIGS. 1A through lJ. Layer thicknesses and through-hole sizes are not representative of either actual or relative scale. For ease of representation of the various steps in the process, a section of the printed circuit board is shown involving, on each side, one through-hole, one pad, and one trace line; the trace will be in association with a different pad and through-hole area on the board (not shown), while the through-hole and pad will be associated with a different trace on the board (not shown).
As shown in FIG. lA, a non-conductive substrate 10, typically an epoxy glass resin, has applied to it on both sides thin copper foil laminate 12. A through-hole 14 has been drilled in the laminated board, and the inner hole surfaces are thus composed of the non-conductive substrate.
In order to provide a conductive connection between the circuitry eventually applied on both sides of the laminate, the through-hole surfaces must be made conductive. As shown in FIG. 1B, the first step in this process is to electrolessly deposit a copper layer 16 on the entirety of the board, i.e., on the through-hole surfaces and on the copper foil 12 (conditioning and activating steps preliminary to copper deposition not shown).
The desired circuit pattern is then applied to the electroless copper layer through application and subsequent exposure and development of a negative photoresist. The areas of the photoresist exposed to light cross-link and become insoluble to developers which remove non-exposed, non-cross-linked areas. As a consequence, there are now present on the elctroless copper layer, exposed copper areas corresponding to traces, pads and through-holes, while remaining areas are covered by material 18 resistant to subsequent plating, as shown in FIG. 1C.
In the next step in the process, copper thickness in the exposed areas is built up through an electroplated copper layer 20 to arrive at the configuration shown in FIG. 1D.
Following copper electroplating, an etch resist 22, generally tin-lead, is electroplated onto exposed copper surfaces as depicted in FIG. 1E. After completion of this step, the plating resist 18 is removed (FIG. 1F) in preparation for copper etching, the etching resulting in the configuration shown in FIG. 1G.
Since the solder mask is to be applied to bare copper, the tin-lead etch resist 22 is stripped away in the next step as shown in FIG. lH. It is now desired to solder coat the pads and through-holes but not the traces and, accordingly, a solder mask 24 is applied to the board in a pattern appropriate to protect all areas where solder is undesired, as shown in FIG. lI. Thereafter, the exposed copper at the holes and pads is cleaned and prepared for solder coating, and then solder coated by, e.g., hot air level solder to present the solder-coated surface 26 as shown in FIG. lJ. Electrolytic processes for application of a solder coat cannot be employed in this method at this stage since the prior step of copper etching has removed the electrical continuity among areas of the board.
As will be readily apparent, the known techniques for solder mask over bare copper, while effective for eliminating problems inherent in application of solder mask to tin-lead coated copper, involve a number of steps which on their face appear almost duplicative but which nevertheless are necessary to gain the advantages of SMOBC. In particular, it will be seen that tin-lead is applied in the normal course of manufacture as an etch-resist over surfaces such as traces, pads and holes, and this tin-lead etch-resist is generally of the same or similar alloy composition as the solder eventually applied to the pads and holes. Nevertheless, the manufacturing sequence requires that this tin-lead etch-resist be stripped and removed so that effective solder masking of bare copper traces can be accomplished. Not only do these additional steps result in increased manufacturing cost, but significant waste removal and pollution concerns arise. Still further, truly flat solder surfaces are difficult to obtain even using hot air leveling.